Field Ionization Mass Spectrometry (FIMS) is well suited to hydrocarbon analysis. For normal paraffins and naphthenes, FIMS may be used to empirically determine the distribution of hydrocarbons as a function of carbon number or molecular weight. Their concentrations can be determined from relative ion abundance or peak intensities in the FIMS spectra. For example, U.S. Pat. No. 5,644,129 teaches the use of FIMS for just such a purpose. FIMS has also been found useful in the study of high molecular weight hydrocarbons. See for example, Field Ionization Mass Spectrometric Study of High Molecular Weight Hydrocarbons in a Crude Oil and a Solid Bitumen, Jose C. de Rio et al, Organic Geochemistry 30 (1999).
FIMS has not, however, been successfully used for the direct determination of the carbon number or molecular weight distribution of isoparaffins.
Conventional electron—impact ionization and even chemical ionization produce extensive fragmentation of isoparaffins, resulting in the absence of molecular or pseudo-molecular ions in the mass spectra. Conventional field ionization techniques such as those used in U.S. Pat. No. 5,644,129 also result in a high percentage of isoparaffin molecular ions being broken apart to form fragment ions. The carbon number distribution of isoparaffins is, therefore, currently estimated by comparing their retention times eluting off a non-polar “boiling point” gas chromatographic column with those of normal paraffins. Due to substantial overlap of components and broad distributions of isoparaffins, such estimation techniques are often not accurate.
This invention has discovered a means for producing intact molecular ions for isoparaffins for direct mass measurement.